The present invention relates to a print head for matrix printers or the like using a carrier for the actuating coils which operate styli for purposes of providing the imprint of individual dots out of which a character to be printed will be composed. A coil carrier of the type to which the invention pertains is provided with magnetic yokes arranged around the circumference of the carrier as a whole whereby each yoke is associated with an electromagnetic actuating coil as well as with an armature which operates directly the respective stylus; the armature abuts the rear end of the stylus and the front end of the latter provides the imprint; selective application of current to the coil followed by suitable retraction operation causes the respective stylus to reciprocate. In particular the invention relates to a print head of the type in which the armature retraction is provided through the operation of a permanent magnet which affects the magnetization of the respective electromagnetic actuating circuit such that there is a reversal of forces whenever current does not flow through the respective coils.
Print heads of the type to which the invention pertains have in the past been used in an environment equivalent to slow, line printers providing roughly 600 characters per second. However, such print heads are considerably less expensive than the comparable line printers as far as print speed is concerned. On the other hand, it was found that the life of such print heads measured in millions of characters printed is lower than a conventional line printer. Therefore, current development tends to improve print heads for matrix printers towards a greater life while, of course, trying to retain the economic advantage over line printers. In this regard it is to be considered that a print head for matrix printers is an object which has a particular mass which reciprocates across the platen. Therefore, the head has to be accelerated from a speed value of zero up to a constant speed of propagation across the platen and at the end it has to be stopped for purposes of reversal. This rather significant exertion of force and the effects of inertia are instrumental in reducing the life of such a head. It is also apparent that these problems are the more pronounced the heavier such a head is constructed.
The German printed patent application No. 30 40 399 proposes a high speed dot-matrix printer generally of the type outlined above and operating on a continuous basis at a speed of 800 characters per second. The head used in that printer is supposedly economical and is definitely lighter because the construction is such that about only one-third as many parts are used as in other known print heads. Moreover, this known printer is particularly constructed with regard to the respective magnetic circuits in that the pole faces of the pole shoes are arranged in planes which are arranged in right angles to each other and the several pole faces are placed in close proximity such that the armature is provided with surfaces respectively running parallel to these pole faces. Moreover, a pair of intersecting, bending members is provided for the formation of a pivot bearing for any of the armatures and the pivot axis is situated at least approximately in the plane of the pole face of the electromagnet. This way a motion is forcedly imparted upon the armature to run along a track which intersects the plane of the pole face of the respective electromagnet so that the permanent magnet included in the circuit holds the armature in abutment with the pole surface of the electromagnet and at a small distance from the pole shoe associated with the respective permanent magnet.
The magnetic system as described in this prior art publication is undoubtedly endowed with certain advantages. However, it includes more air gaps in the magnetic circuit as is desired. Moreover, the construction of each of the several components involved is quite extensive and expensive; reference is made here particularly to the construction involving intersecting or crossed bending members for purposes of establishing a particular kind of pivot bearing for the armature.